Abstract
Pooled milk is used for the surveillance of several diseases of livestock. Previous studies demonstrated the detection of foot-and-mouth disease virus (FMDV) in the milk of infected animals at high dilutions, and consequently, the collection of pooled milk samples could be used to enhance FMD surveillance. This study evaluated pooled milk for FMDV surveillance on a large-scale dairy farm that experienced two FMD outbreaks caused by the A/ASIA/G-VII and O/ME-SA/Ind-2001d lineages, despite regular vaccination and strict biosecurity practices. FMDV RNA was detected in 42 (5.7%) of the 732 pooled milk samples, and typing information was concordant with diagnostic reports of clinical disease. The FMDV positive milk samples were temporally clustered around reports of new clinical cases, but with a wider distribution. For further investigation, a model was established to predict real-time RT-PCR (rRT-PCR) CT values using individual cattle movement data, clinical disease records and virus excretion data from previous experimental studies. The model explained some of the instances where there were positive results by rRT-PCR, but no new clinical cases and suggested that subclinical infection occurred during the study period. Further studies are required to investigate the effect of vaccination on FMDV excretion in milk, and to evaluate more representative sampling methods. However, the results from this pilot study indicate that testing pooled milk by rRT-PCR may be valuable for FMD surveillance and has provided evidence of subclinical virus infection in vaccinated herds that could be important in the epidemiology of FMD in endemic countries where vaccination is used.
Highlights
Milk has been exploited for the surveillance of several pathogens of livestock including bovine viral diarrhea virus [1, 2], Schmallenburg virus [3], Coxiella burnetti [4], bovine respiratory syncytial virus [5], and Neospora caninum [6]
During the study period 732 milk samples were collected of which 42 (5.7%) were positive using the pan-serotypic reverse transcription polymerase chain reaction (rRT-PCR) (Table 2, Figure 3B). Of these positive samples (n = 42), and for those not considered positive but had very low amplification below the fluorescence threshold of 0.2 (“inconclusive,” n = 22), 32.8% were positive by the A/ASIA/G-VII rRT-PCR assay, and 9.4% were positive by the O/ME-SA/Ind-2001d rRT-PCR assay (Figure 3C, Supplementary Data File 2)
This study aimed to expand on previous work to determine the utility of testing pooled milk by rRT-PCR as an alternative approach for FMD surveillance in vaccinated dairy herds
Summary
Milk has been exploited for the surveillance of several pathogens of livestock including bovine viral diarrhea virus [1, 2], Schmallenburg virus [3], Coxiella burnetti [4], bovine respiratory syncytial virus [5], and Neospora caninum [6]. Previous studies [9, 13] have suggested that it could be possible to identify one acutely-infected milking cow in a typical-sized dairy herd (100–1,000 individuals) using milk from bulk tanks or milk tankers. This theory was based on the detection of FMDV RNA in milk samples, collected from infected cattle, that had been highly diluted over 10,000-fold in negative milk. Simulation modeling using these data [13, 15, 16] support the requirement for further research to assess the use of pooled milk as a useful tool to enhance FMD surveillance
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